The present invention relates to a belt unit provided with a reading section that reads a toner image borne on a surface of an endless belt and to an image forming apparatus.
Among image forming apparatus according to the electrophotography method is one that is provided with a belt unit transferring a toner image using the endless belt to an object to which the transferring is made.
Due to environmental variations such as temperature, humidity and/or the like and deterioration with passage of time such as degradation of an image bearing member and/or degradation of the toner, it is made impossible for the belt unit to maintain a pertinent concentration of the toner image that is borne on the endless belt, which occasionally results in image quality failure. In this regard, as disclosed in the Japanese Patent Unexamined Publication No. 2008-241958 bulletin, among the image forming apparatus each provided with a belt unit is one in which concentration of the toner image is adjusted by forming a reference toner image onto the endless belt with a predetermined imaging condition, then reading the concentration of the reference toner image from the surface of the endless belt by the reading section, and then changing the imaging condition so that the concentration that is read becomes a predetermined reference concentration. In order for the reading section to read the concentration of the reference toner image accurately, it is necessary to prevent flapping of the endless belt while keeping a gap between the reading section and the surface of the endless belt constant; therefore, when the reading is carried out by the reading section, a supplementary roller is caused to be in contact with a rear face of the endless belt.
Further, the reading section sometimes includes means for detecting meandering quantity of the endless belt for use in meandering adjustment of the endless belt.
Yet, because the endless belt is curved at a portion that is in contact with the supplementary roller, toner's falling from the curved surface can occasionally occur. If the falling toner sticks to a reading face, it follows that the reading section becomes unable to read the toner image from the surface of the endless belt accurately.
The present invention is directed to providing a belt unit that can prevent the toner falling from the endless belt from sticking to the reading face of the reading section, and an image forming apparatus.